1. Field of the Invention
The present invention relates to a method for determining the incidence of a signal from an acoustic signal source using a directional microphone system, which has at least two microphones, and to an apparatus for implementing the method.
2. Description of the Prior Art
For various acoustic signal-processing algorithms, it is necessary to know the direction of incidence of an incoming signal. This is important, for example, in the case of controllable directional microphone systems which allow interference signals to be deliberately suppressed. For this purpose, for example, microphones in a directional microphone system are optimized with regard to the direction of incidence by a matching process in such a way that they eliminate the interference signal component in the directional microphone signal as much as possible.
Furthermore, localization and frequency determination are important for acoustic scene analysis (Auditory Scene Analysis, ASA). In an analysis such as this, for example, an angle at which a signal is incident, a frequency of the signal or else a distance from the signal source are determined.
Various methods, which are based on correlation analysis between microphone signals, which have been delayed for different periods, exist for determination of the direction of incidence. The direction of incidence is estimated as a function of the correlation and of the geometry of the microphones. These methods, however, are rather unsuitable for small microphone separations, such as those for single ear headsets since, in this case, the maximum delay time between two microphones is typically less than one sampling period. If the sampling rate is 20 KHz sound is returned in a sampling period of about 17 mm. These methods are inaccurate for typical microphone separations in directional microphone systems of 8 to 12 mm.
In addition, so-called head-related transfer functions (HRTFs) come into play, for example when using the directional microphone in a hearing aid that is worn on the head. These corrupt the result of the direction estimation for the localization of the signal source in comparison to the situation of a directional microphone that is used in the open air and is not acoustically influenced by the environment. A further disadvantage of direction estimation with two or more microphones arranged on one axis is that the direction cannot be determined unambiguously in the open air, since there is always a symmetrical result about the microphone axis. This means that an estimate of the direction of incidence such as this is not unambiguous.
A method for matching microphones in a hearing aid, as well as a hearing aid operating according to the method, are known from German PS 199 27 278. In this case, a hearing aid with two or more microphones which are connected to one another in order to produce a directional characteristic is ensonified while being worn in a suitable measurement area, and the directional characteristic is recorded. Filter parameters which result from this can be supplied to configurable filters which are connected downstream from the microphones and it is thus possible to approximate the desired ideal directional characteristic taking account of the individual characteristics when the hearing aid is being worn. This method allows filter parameters to be produced for amplitude and phase response matching of signals recorded by the microphones, in order to optimize the directional characteristic of the microphones.